1. Field of the Invention
The present invention relates to ram air parachutes and more particularly to ram air parachutes having an improved canopy design.
2. Discussion of Related Art
Parachutes have evolved over the years into highly sophisticated systems, and often include features that improve the safety, maneuverability, and overall reliability of the parachutes. Initially, parachutes included a round canopy. A skydiver was connected via a parachute harness to the canopy by suspension lines disposed around the periphery of the canopy. Such parachutes severely lacked control. The user was driven about by winds with little mechanism for altering direction. Furthermore, such parachutes had a single descent rate based upon the size of the canopy and the weight of the parachutist. They could not generate lift and slowed descent only by providing drag.
In the mid-1960""s the parafoil canopy was invented. Since then, variations of the parafoil canopy have replaced round canopies for most applications, particularly for aeronautics and the sport industry. The parafoil canopy, also known as a ram air canopy, is formed of two layers of materialxe2x80x94a top skin and a bottom skin. The skins may have different shapes but are commonly rectangular or elliptical. The two layers are separated by vertical ribs to form cells. The top and bottom skins are separated at the lower front of the canopy to form inlets. During descent, air enters the cells of the canopy through the inlets. The vertical ribs are shaped to maintain the canopy in the form of an airfoil when filled with air. Suspension lines are attached along at least some of the ribs to maintain the structure and the orientation of the canopy relative to the pilot. The canopy of the ram air parachute functions as a wing to provide lift and forward motion. Guidelines operated by the user allow deformation of the canopy to control direction and speed. Ram air parachute canopies have a high degree of maneuverability.
Canopies are flexible and stretchable membrane structures, they distort based upon mechanical and aerodynamic tensions, stresses, airflows and pressure distribution. Although a cell is modeled as having a basically rectangular cross section, when inflated the shape distorts towards round with complex distortions. Under canopies of conventional design, the leading edge or nose of the ram air parachute is deformed during flight as is the top profile of the airfoil between the ribs. Additionally, with forward motion, the head-on wind overcomes the internal pressurization of the canopy, and deforms the nose of the canopy. This distortion blunts the nose of the airfoil or even indents it, impairing the aerodynamics of the parachute wing. The parachute flies less efficiently as a result. Therefore, a need exists for a ram air parachute canopy which reduces nose distortion and spanwise topskin distortion.
Inlets are required to inflate and pressurize the canopy to maintain its wing shape. However, the inlets are also the greatest source of drag on the wing which slows forward movement and reduces efficiency. The carrying capacity and glide ratio of the canopy would be improved if this drag could be reduced. Therefore, a need exists for a canopy with reduced drag from the cell inlets.
Typically, in a ram air parachute, suspension lines are attached to every other rib, thus creating loaded ribs (i.e., ribs to which suspension lines are attached) and non-loaded ribs (i.e., ribs which do not have suspension lines attached thereto). The different stresses on the loaded and non-loaded ribs also distorts the cell shape. FIG. 1 illustrates a cross section of a portion of a typical ram air parachute canopy 500 during flight. FIG. 1 shows two cells formed of parts 501, 502, 503, 504, with three loaded ribs 510, 511, 512 and two non-loaded ribs 521, 522. Suspension lines 541, 542, 543 are attached to the loaded ribs 510, 511, 512. The top skin 530 and bottom skin 531 tend to arc between the ribs during inflation. Also, the non-loaded ribs 521, 522 tend to be higher than the loaded ribs 510, 511, 512, which provides a distortion along the span of the canopy. The distortion is aerodynamically undesirable and reduces the efficiency and performance of the canopy.
In order to keep the loaded and non-loaded ribs level and to improve upon the aerodynamics of the canopy, cross-bracing between ribs has been added to some canopy designs. Cross bracing is the use of diagonal ribs in addition to vertical ribs to create more loaded ribxe2x80x94top skin junctions without adding more lines which increase drag and possible deployment malfunctions. Perfection of the top profile of the airfoil is far more important aerodynamicly than the bottom profile. U.S. Pat. No. 4,930,927 illustrates such a design. Cross-braced designs suffers from a number of drawbacks. Cross-bracing results in very complicated construction, high manufacturing costs, and increased packing volume. The standard cross braced design is a xe2x80x98tri cellxe2x80x99 construction with a packing volume approximately 25% larger than an equivalent non-cross braced design. A cross section of a tri-cell canopy is illustrated in FIG. 2. Furthermore, the increased rigidness induced by the cross-bracing creates higher opening forces for the pilot. Typically, large cross porting is used on all of the cells to reduce pack volume, which does nothing to slow the canopy""s inflation on deployment. The opening forces can be so severe that they can jar the jumper""s body causing discomfort and even injuries. Although designers have implemented xe2x80x9cformedxe2x80x9d noses, larger sliders, moved bridal attachment points and modified line trims to try to soften the openings of such cross-braced canopies, it has generally yielded limited improvement.
Sliders used to counteract the large opening forces on a cross-braced canopy often cause premature wear on the suspension lines of the canopy. A slider is a rectangular piece of material with a grommet at each corner. Grouped suspension lines pass through each grommet. When the parachute opens, the force of the opening canopy and separating suspension lines forces the slider down the suspension lines. Air resistance tends to slow movement of the slider and, hence, restrict opening of the canopy against the spreading force of the inflating canopy pushing the slider down. The most force on the slider comes from the lines to the outermost cells, which pushes the slider down rapidly caused friction heat. The heat changes the dimension of many standard types of lines (e.g., Spectra, dyneema brand lines). It is not uncommon for outer lines to change in dimension as much as five inches in only a couple of hundred jumps. Accordingly, cross braced canopies are almost exclusively supplied with Aramid based lines (e.g., Kevlar, Vectran, etc.). These lines do not change dimension with the generated slider-friction heat solving the problem stated above, but suffer from micro-fiber cracking. Accordingly, if over jumped, Aramid lines can break catastrophically with no warning.
Prior art canopies have included formed noses with shaped inlets to limit opening forces. FIGS. 3 and 4 illustrate two prior art canopy designs having a formed nose. FIG. 3 illustrates a tri-cell design with an formed nose having an oval inlet 801 by a loaded rib 802. Reinforcing tape 803 is sewn around the oval shape of the nose. FIG. 4 illustrates a formed nose created by extending the top skin 810 and bottom skin 811 around the nose of the canopy to create shaped inlets 812. Again, reinforcing tape 813 is sewn to the inlet edges. While the fabric tape on the inlet edges of the formed noses in these designs limits wear, the canopy is still subject to span-wise stretching. The entire span of the canopy will stretch during flight and span wise distortion of the nose occurs due to the different stresses on loaded and non-loaded ribs. All prior art canopies with formed noses place the open inlets over a loaded rib. This creates a geometry which, during deployment, presents scoops for inrushing air. The scoop shape results in large opening forces and in substantial drag during flight.
Accordingly, there is a need for an improved parachute airfoil design which provides ram air parachutes with lower spanwise distortion without using crossbraces and with restricted inlet area for softer openings. Furthermore, a need exists for a canopy design which limits distortion and stretching at the leading edge for improved aerodynamics with formed nose inlets. A need exists for a packable canopy design which provides improved aerodynamics with limited packing volume
The deficiencies of the prior art are substantially overcome by the canopy design of the present invention which reduces spanwise distortion on the top skin without using cross braces. According to one aspect of the invention, the size of the inlets are changed to reduce drag on the canopy and control the conductance of air inflating the parachute during deployment. according to a first embodiment of the invention, triangular pieces of material are attached to the front edge of the top skin at each of the loaded ribs. The triangular material reduces the inlet size and thus drag on the canopy during flight. The triangular material also tensions the top skin between the loaded ribs and the non-loaded ribs to reduce spanwise distortion. With the addition of this triangular patch on the nose at each rib inlet pack volume is increased by only a miniscule amount, but tension is now transmitted from the suspension line-loaded rib junction through the patch and its reinforcing tape to the top skin of the canopy cell greatly reducing the spanwise distortion of the nose. The distortion is reduced to the same amount present with crossbracing only at the nose of the wing and then tapers back to a usual amount about ⅓ of the way back on the airfoil. Aerodynamically this is the most important portion of the wing. With equal dimensioned triangle patches on both the loaded and non loaded ribs the nonleaded rib triangles will not tension in the same way than at the loaded ribs and tend to remain slack. Such a design performs well over prior art but can be further improved by changing the shape of the nonloaded triangles to have a broader angle off the vertical rib so as to overlap with the loaded rib triangles. In this way side they are side tensioned from the loaded rib triangle edges.
According to a second embodiment of the present invention, triangular pieces of material are added to the front edge of the top skin at the loaded and non-loaded ribs. According to another embodiment of the present invention, the top or bottom skin of the canopy are modified to provide the shape of the triangular pieces of material of the other embodiments. The edges of the formed inlet opening are reinforced with woven fabric tape. The resulting configuration looks akin to a triangulated reinforced truss.
According to a third embodiment of the a line attachment points of the canopy are set back slightly from the nose of the canopy and the bottom skin is cut with a zig-zag or scalloped edge. In this embodiment, the additional inlet area is formed that is only presented during deployment when the relative wind is from below. Once inflated with the canopy in gliding flight the relative wing is from slightly below head on. Prior art canopies use inlets that when their area is projected in the direction of the relative wind have more projected inlet area during flight than during opening. this is reversed from ideal in that more inlet area is required for opening than is required to keep the canopy pressurized for flight. Any additional inlet area than required for flight is simply additional drag. By undercutting the bottom skin the inventive method can ensure proper deployment of canopies with very low drag restricted inlets for flight.